Method and apparatus for uniformly stretching thermoplastic film and products produced thereby

ABSTRACT

Thermoplastic films and laminates are made by extrusion and stretching using differential speed rollers employing a short stretching gap of no more than one inch immediately after the extruded film product is formed. The method and apparatus control the thickness of the stretched film and impart desirable mechanical properties thereto such as film products with a high modulus for handling and suitable tensile for softness.

RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.10/838,920, filed May 4, 2004.

TECHNICAL FIELD OF THE INVENTION

This invention is directed to a method for uniformly stretching athermoplastic film, thereby controlling thickness of the stretched filmand imparting desirable mechanical properties thereto.

BACKGROUND OF THE INVENTION

There are three common ways to stretch thermoplastic films. One isreferred to as machine direction orientation (MDO) which involvesstretching the film between two pairs of rollers. The film is pinched inthe nip of a first pair of rollers, which are running at a relativelyslow speed, and a second pair of rollers, downstream from the firstpair, which are operating faster than the first pair. Because of thedifference in run speeds, the film in between the roller pairs musteither stretch or break to accommodate the difference. The ratio of theroller speeds will roughly determine the amount that the film isstretched. For example, if the first pair is running at 100 feet perminute (fpm) and the second pair is running at 300 fpm, the film will bestretched to roughly three times it original length. The MDO methodstretches the film in the machine direction (MD) only. The MDOstretching method is used to create an oriented film or a microporousfilm, for example, where the film contains a dispersed inorganic fillerwhich creates microporosity upon stretching. The micropores formed inthe microporous film as stretched by MDO tend to be oval and arerelatively large, for example, up to 1.5 μm microns on the long axis. Anearly example of stretching or orienting a microporous film to improvegas and moisture vapor transmission by differential speed rollers isU.S. Pat. No. 3,832,267 which issued in 1974. This '267 patent alsodiscloses a second method of stretching which is called tentering. Insimplest terms, the tentering method involves grabbing the sides of thefilm and stretching it sideways. For many years this was the only way tostretch film from side to side, or in the cross direction (CD). Thetentering method tended to be slow and, because the forces areconcentrated on the edges of the film, often the film did not stretchevenly. U.S. Pat. No. 4,704,238 discloses a tentering apparatus having apre-heating zone and a stretching zone, followed by a heat setting zoneto facilitate the stretching of a preformed blown or cast film. In this'238 patent, the thermoplastic film contains inorganic fillers such ascalcium carbonate which, when stretched by tentering and/or heattempering, produces a microporous film. Another example of MDOstretching of polymeric film and multilayered film is EP 848663 where anextruded film product may first be cooled, and then later heated andstretched to form a breathable film product. Additionally, the extrudedfilm may be stretched immediately after extrusion, before it is cooled.

A third method of stretching involves incremental stretching ofthermoplastic film. This method is described in the early patentliterature, for example, U.S. Pat. Nos. 4,153,751; 4,116,892; 4,289,832and 4,438,167. In the practice of this method, the film is run betweengrooved or toothed rollers. The grooves or teeth on the rollersintermesh without touching when the rollers are brought together and, asthe film passes between the rollers, it is stretched. Incrementalstretching has the advantage of causing the film to stretch in manysmall increments that are evenly spaced over the entire film. Thisresults in a more evenly stretched film, something that is not alwaystrue for MDO stretching and is almost never true for tentering.Incremental stretching allows one to stretch the film in the MD, CD andat angle or any combination of these three directions. The depth atwhich the intermeshing teeth engage controls the degree of stretching.Often, this incremental method of stretching is simply referred to asCD, MD or CD/MD. A number of U.S. patents have issued for incrementallystretching thermoplastic films and laminates. An early example of thepatent art which discloses a method of incrementally stretching film isU.S. Pat. No. 5,296,184. Other relevant patents regarding theincremental stretching of thermoplastic films and laminates include U.S.Pat. Nos. 6,265,045; 6,214,147; 6,013,151; 5,865,926; 5,861,074;5,851,937; 5,422,172 and 5,382,461.

The above brief description of stretching techniques and apparatus toproduce an oriented or stretched polymeric film illustrates the effortsthat have been made to produce film products having desirable aestheticand mechanical properties. Notwithstanding these efforts, there is anongoing effort to improve known methods for producing thermoplasticfilms and laminates thereof to achieve quality products having improvedproperties. Moreover, there is a continuing effort to improve apparatusfor producing oriented or stretched thermoplastic film withoutsignificant capital expenditures. It has been a very desirable objectiveto make improvements with savings in capital expenditures and processingcosts, yet with production of quality products.

SUMMARY OF THE INVENTION

This invention is directed to a method of making a stretchedthermoplastic film having substantially uniform gauge with improvedmechanical properties. The method involves extruding the thermoplasticextrudate in the form of a web in its molten state and locatingdifferential speed rollers to chill the film to its solid state andstretching the film between the rollers to achieve a stretched filmhaving a substantially uniform gauge.

It has been found critical in accordance with the principles of thisinvention, in order to achieve its objectives and advantages, to locatea second roller downstream from the first chill roller to provide astretch or roll gap of no more than 1 inch (simply hereinafter “shortgap”) between the rollers for stretching the film. It has been found ifthe short gap is not maintained, that thermoplastic film products havingthe desired mechanical properties and aesthetics are not obtained. Forexample, the short gap apparatus and method according to this inventionenable the production of film products where the gauge or thickness issubstantially uniformly maintained. This control over film thicknesseliminates what is referred to in the trade as “tiger stripes” which arevisual evidence of nonuniformity in thickness which renders the filmsomewhat ugly and aesthetically undesirable, especially since the filmproducts are used in the fabrication of clothing where its unsightlinessis undesirable. In addition, film products are desired where modulus andtensile strengths need to be maintained in order to have usefulproducts.

The desired objectives of this invention are achieved by utilization ofthe residual extrusion heat upon film formation to facilitate thestretching immediately after film extrusion. However, this desirablesavings of heat energy will not necessarily result in the production ofa satisfactory product, unless the short gap for film stretching isutilized. Experimental data in the detailed operating examples of thisinvention, as hereinafter described, illustrate the practice of thisinvention and the obtainment of the desired results.

This invention enables the conversion of a conventional cast extrusionline to an MDO processing line without making a capital expenditure forconventional MDO equipment. In addition, the present invention reducesenergy costs because the extra step of heating the cooled web in atypical MDO stretching process is eliminated.

This invention also enables a conventional cast extrusion line toproduce polymer film at a rate of speed exceeding the limits of theextruder. For instance, one limitation to the speed at whichthermoplastic polymers can be extruded is the onset of draw resonance asthe extrudate exits the extruder die. If a given extruder can produceextrudate at a maximum rate of 1000 linear feet per minute withoutresonance, this invention makes it possible to stretch the resultingproduct without draw resonance in order to produce 2000, 3000, 4000, oreven more linear feet per minute (fpm) at the winder. Thus, in thisembodiment of the invention, the velocity (V₁) of the extrudate isslightly below that where the onset of draw resonance in the extrudedweb occurs. The extruded film at V₁ having a certain original thicknessis then stretched in the short stretch gap at a velocity (V₂) which isin the range of up to about 4, i.e., about 2 to 4, or more, times thevelocity V₁. The resulting film product has a gauge which is thinner bycorresponding ratios of, for example, about 2 to 4 or more times thinnerthan the original extruded film. Accordingly, commercially acceptablethin films of substantially less than 1 mil, e.g., 0.4 to 0.6 mil, ofuniform gauge can be produced at high speeds of 2000 to 4000 fpm, ormore, by the method and apparatus of this invention.

From a compositional standpoint, in brief, the thermoplastic filmproducts comprise a polymer consisting of polyolefin, polyester, nylonand blends or coextrusions of two or more of such polymers. Where amicroporous film product or laminate is desired, the thermoplasticextrudate contains a dispersed phase of pore-forming particles which maybe an inorganic filler or organic particles. Depending upon the polymerand desired degree of microporosity to achieve breathability, thetemperature of the film and the short gap between the rollers is in therange of between 20° to 100° C.

One of the objectives of the invention is to achieve a film product witha high modulus for handling and suitable tensile for softness.Elastomeric polymers may also be used as the thermoplastic extrudateincluding metallocene polymers involving copolymerization of ethylenewith an alpha-olefin comonomer such as butene, hexene or octene.

The invention is also directed to an apparatus for making a stretchedthermoplastic film comprising an extruder for extruding thethermoplastic extrudate and differential speed rollers to chill the filmand provide a roll gap of no more than 1 inch for stretching the film toprovide a film of uniform gauge thickness. The chill roller may comprisean embossing metal roller or a flat chrome roller, typically incombination with the backing of a rubber roller or metal roller toachieve desirable film properties and aesthetics such as an embossedcloth-like surface, matte finish or other textures. The backing rollercooperates with the chill roller to form a nip for receiving the moltenweb and facilitating stretching. Similarly, the second roller whichoperates at a peripheral speed higher than the chill roller tofacilitate stretching in the short gap may cooperate with anotherbacking roller in order to form a second nip for receiving the film fromthe first nip. Typically the second roller is a metal roller and thebacking roller is a rubber roll.

The apparatus for forming the short gap can be combined with incrementalstretching rollers which further treat the stretched film byincrementally stretching it. Incremental stretching of the film modifiesthe mechanical properties of the film that have been achieved bystretching the film in the short gap. For instance, the various texturesand aesthetics may be modified by a further incremental stretching whichaffects the moisture vapor transmission rate (MVTR) and airbreathability of the film. The incremental stretching rollers areusually comprised of a first section and a second section forincrementally stretching the film in a first direction (MD) followed byincrementally stretching in a second direction (CD), preferablysubstantially perpendicular to the MD.

The following detailed description and examples illustrate the method ofmaking the thermoplastic film products of this invention and theapparatus. In light of these examples and this detailed description, itis apparent to a person of ordinary skill in the art that variationsthereof may be made without departing from the scope of the invention.Furthermore, other benefits, advantages and objectives of this inventionwill be further understood with reference to the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is further understood with reference to the drawings inwhich:

FIG. 1 is a schematic of an in-line extrusion process with MDOstretching and incremental stretching for making the film or filmlaminate of this invention using the short stretching gap “x”;

FIG. 2 is a cross-sectional view taken along 2-2 of FIG. 1 illustratingintermeshing rollers in diagrammatic form;

FIG. 3 is a schematic of an alternative form of the MDO stretchingapparatus with the short gap “x”

FIG. 4 is a SEM photomicrograph of an unstretched film of Example I-O;

FIG. 5 is a SEM photomicrograph of a stretched film of Example I-A;

FIG. 6 is a SEM photomicrograph of a stretched film of Example I-B;

FIG. 7 is a SEM photomicrograph of a stretched film of Example I-C;

FIG. 8 is a SEM photomicrograph of a stretched film of Example I-D;

FIG. 9 is a SEM photomicrograph of a stretched film of Example II-A;

FIG. 10 is a SEM photomicrograph of a stretched film of Example II-B;

FIG. 11 is a SEM photomicrograph of a stretched film of Example II-C;

FIG. 12 is a SEM photomicrograph of a stretched film of Example III-A;

FIG. 13 is a SEM photomicrograph of a stretched film of Example III-B.

FIG. 14 is a schematic illustrative comparative exhibit of Examples V-Ato V-D.

DETAILED DESCRIPTION OF THE INVENTION

It is one objective of this invention to convert a conventional castextrusion line to make a stretched MDO thermoplastic film productwithout making a capital expenditure for MDO equipment which isconventionally used to perform MDO processing. In addition, it is anobjective of the present invention to save money and energy in theproduction of thermoplastic film and laminate products. It is also anobjective of this invention to simplify the processing and stretching ofthermoplastic film by elimination of extra steps such as heating a coolweb in a typical MDO process. It is also an objective of this inventionto produce thermoplastic film at a rate of speed in excess of thelimitations of the extruder. One of the benefits of this invention isthe obtainment of a stretched thermoplastic film having a substantialuniformly gauge and desirable mechanical properties such as tensile,impact and modulus. Thermoplastic films and laminates are producedaccording to the invention which are aesthetically desirable, i.e.,having a cloth-like appearance and feel. Microporous film productshaving significant moisture vapor transmission rates are also produced.

The above objectives and benefits of this invention are achieved, withreference to FIG. 1 which shows an in-line film extrusion laminationsystem with MDO stretching and, optionally, a CD+MD interdigitatingroller section. The conventional film extrusion system involves anextruder 21 and a die 22. Thermoplastic web 26 is extruded in its moltenstate from the die 22 past an air knife 23 into a nip which involves arubber roller 25 and metal roller 24. When a nonwoven and film laminateis desired, the nonwoven 20 is supplied to web 26. The surface of rubberroller 25 is typically cooled by immersing it partially in a water tankwhich is not shown. The film 26 is cooled by an air knife 23 of FIG. 1or by an air knife 33 and vacuum box 34 which operates in conjunctionwith die 35 and metal rollers 36, 37 as shown in the alternativeembodiment of FIG. 3. The metal roller 24 can be an engraved pattern ora flat chrome roller to produce either embossed or flat film and it iscooled to a specific temperature after passing through the nip of themetal roller 24 and rubber roller 25. The metal roller 24 serves as achill roller which is typically controlled at a temperature (T₁) toreceive and cool the web to its molecularly orientable and stretchablestate. The second roller 27 is also typically maintained at atemperature (T₂) which is at or lower than T₁. The temperatures T₁ andT₂ are maintained depending upon the film properties required, and aretypically in the range of between about the glass transition temperature(Tg) and the melting temperature (Tm) of the polymer present at thehighest concentration in the film. The film is uniformly stretched inthe roll gap x at a stretch temperature of between T₁ and T₂ at a speedbetween V₁ and V₂ to form the stretched thermoplastic film having asubstantially uniform gauge.

The thermoplastic web 26 will be formed flat or embossed and cooled to aspecific temperature after passing over the metal roller 24 which istypically controlled at a temperature T₁ between Tg and Tm of the mainpolymer (for example between 70-160° F. for polyolefin compositions ofPE, LLDPE, HDPE or PP) and stripped away by passing through anothermetal roller 27 which is typically controlled at a temperature T₂ at orbelow T₁ on the film property required. At this point, the film 29 isformed and cooled, and will go downstream where it can be wound into aroll form for different applications. Where incremental stretching isdesired, the MDO stretched film 26 is passed through intermeshingrollers 40, 41 and 42,43 for stretching in the MD and CD, respectively.A rubber roller 28 can be added to form another nip with the metalroller 27. This nip formed by rubber roller 28 and metal roller 27 canbe adjusted horizontally or vertically to form a gap x between the nipof metal roller 24. The gap between the metal rollers 24 and 27 is at0.005″ to 1″, and the metal roller 27 usually runs at about 1.5 to 5times faster than the metal roller 24. The ratio of the speed of themetal roller 27 in feet per minute to the metal roller 24 is defined asthe stretch ratio. The short roll gap “x” is defined as the narrowestdistance between the circumferences of the chill roller 24 and secondroller 27. This distance is measured directly by a feeler gauge which isreadily available.

The roll gap is no more than 1 inch, more preferably about 0.005 toabout 0.05 inch, usually about 0.01 inch. The ratio of V₂ to V₁ providesa stretching ratio between about 1.25:1 and about 4:1. Preferably, thechill roller is a metal roller, such as an embossing roller, whichcooperates with a backing roller to form a first nip for receiving webextrudate. Where nonembossed or flat films are desired, the chill rolleris a flat chrome roller. The second metal roller cooperates with anotherbacking roller (usually rubber) to form a second nip for receiving saidfilm from said first nip. A vacuum box or air knife may be used withsaid chill roller.

A. Thermoplastic Polymers for the Film or Laminates

When a microporous film product is desired, the thermoplastic extrudateis a thermoplastic polymer containing a dispersed phase of pore-formingparticles selected from the group consisting of inorganic filler andorganic material and the stretched thermoplastic film is microporoushaving a moisture vapor transmission rate (MVTR) and being a barrier tothe passage of liquid. An MVTR greater than about 1,000 g/m²/day,according to ASTM E96(E) is achieved, preferably greater than about1,000 to 4,000 g/m²/day, according to ASTM E96(E). More broadly, thethermoplastic extrudate comprises a polymer wherein said polymer isselected from the group consisting of polyolefin, polyester, nylon, andblends or coextrusions of two or more of such polymers. Preferably, thepolyolefin is selected from the group consisting of polyethylene (PE),polypropylene (PP), copolymers thereof, and blends thereof, wherein thepolyolefin contains a dispersed phase of pore-forming particles selectedfrom the group consisting of inorganic filler and an organic material,and said stretched thermoplastic film is microporous. The pore-formingparticle filler is selected from the group consisting of calciumcarbonate, barium sulfate, silica, talc, and mica.

When employing polyolefins to make a film or microporous product of theabove type, i.e., PE, PP, LLDPE, LDPE or HDPE, the temperature of thefilm in said roll gap is in the range of about 20° to 100° C. (68° F. toabout 212° F.), usually in the range of about 30° C. to 80° C. (86° F.to 176° F.). The temperature of said second roller is from about 21° C.to 82° C. (70° F. to 180° F.). The stretching temperature is controlledby a chill roller and the second roller controls T₂ at ambienttemperature or a higher temperature which maintains the film in itsmolecularly orientable and stretchable state. In other words, T₂ ismaintained at or below T₁. Film having a ratio of MD tensile strength at25% elongation to CD tensile strength at 25% elongation of greater than2 can be made which provides it with a modulus for web handling and CDtensile for softness.

Other thermoplastic polymers can be used which are elastomeric.Elastomeric polymers are selected from the group consisting ofpoly(ethylene-butene), poly(ethylene-hexene), poly(ethylene-octene),poly(ethylene-propylene), poly(styrene-butadiene-styrene),poly(styrene-isoprene-styrene), poly(styrene-ethylene-butylene-styrene),poly(ester-ether), poly(ether-amide), poly(ethylene-vinylacetate),poly(ethylene-methylacrylate), poly(ethylene-acrylic acid),poly(ethylene butylacrylate), polyurethane,poly(ethylene-propylene-diene), and ethylene-propylene rubber. Thisclass of rubber-like polymers are generally referred to herein aspolyolefins produced from single-cite catalysts. The most preferredcatalysts are known in the art as metallocene catalysts wherebyethylene, propylene, styrene and other olefins may be polymerized withbutene, hexene, octene, etc., to provide elastomers suitable for use inaccordance with the principles of this invention. The elastomericpolymer also may contain pore-forming inorganic filler particles whichprovide microporosity to said film upon stretching. Stretchedthermoplastic film having a thickness on the order of about 0.25 toabout 10 mils (about 6 to 250 gsm) can be produced, preferably about0.25 to 2 mils (about 6 to 50 gsm).

A preferred thermoplastic extrudate comprises about 30% to about 45% byweight of a linear low density polyethylene (LLDPE) or polypropylene(PP), about 1% to about 10% by weight of a low density polyethylene(LDPE), and about 40% to about 60% by weight of calcium carbonate fillerparticles. The composition can further contain a component selected fromthe group consisting of high density polyethylene (HDPE), titaniumdioxide, and mixtures thereof. The thermoplastic extrudate can be acoextruded structure of one or more layers of different compositions,such as one or more layers of a polymer selected from the groupconsisting of polypropylene, LLDPE, LDPE, and blends thereof.

As stated above, both embossed and flat films may be produced accordingto the principles of this invention. In the case of an embossed film,the nip of rollers comprises a metal embossing roller and a rubberroller. The compressive force between the rollers forms an embossed filmof desired thickness on the order of about 0.25 to about 10 mils.Rollers which provide a polished chrome surface form a flat film.Whether the film is an embossed film or a flat film, upon incrementalstretching, at high speeds, microporous film products are producedhaving high moisture vapor transmission rate (MVTR) within theacceptable range of about 1000 to 4000 gms/m²/day. Laminates of themicroporous film may be obtained with nonwoven fibrous webs. Thenonwoven fibrous web may comprise fibers of polyethylene, polypropylene,polyesters, rayon, cellulose, nylon, and blends of such fibers. A numberof definitions have been proposed for nonwoven fibrous webs. The fibersare usually staple fibers or continuous filaments. The nonwovens areusually referred to as spunbond, carded, meltblown, and the like. Thefibers or filaments may be bicomponent to facilitate bonding. Forexample, a fiber having a sheath and core of different polymers such aspolyethylene (PE) and polypropylene (PP) may be used; or mixtures of PEand PP fibers may be used. As used herein “nonwoven fibrous web” is usedin its generic sense to define a generally planar structure that isrelatively flat, flexible and porous, and is composed of staple fibersor continuous filaments. For a detailed description of nonwovens, seeNonwoven Fabric Primer and Reference Sampler by E. A. Vaughn,Association of the Nonwoven Fabrics Industry, 3d Edition (1992).

In a preferred form, the microporous laminate employs a film having agauge or a thickness between about 0.25 and 10 mils (a weight of 6 to250 gsm) and, depending upon use, the film thickness will vary and, mostpreferably, in disposable applications is the order of about 0.25 to 2mils in thickness. The nonwoven fibrous webs of the laminated sheetnormally have a weight of about 5 gms/yd² to 75 gms/yd², preferablyabout 20 to about 40 gms/yd². The composite or laminate can beincrementally stretched in the cross direction (CD) to form a CDstretched composite. Furthermore, CD stretching may be followed bystretching in the machine direction (MD) to form a composite which isstretched in both CD and MD directions. As indicated above, themicroporous film or laminate may be used in many different applicationssuch as baby diapers, baby training pants, catamenial pads and garments,and the like where moisture vapor and air transmission properties, aswell as fluid barrier properties, are needed.

B. Stretchers for the Microporous Film and Laminates

A number of different stretchers and techniques may be employed tostretch the film or laminate of a nonwoven fibrous web andmicroporous-formable film. These laminates of nonwoven carded fibrouswebs of staple fibers or nonwoven spun-bonded fibrous webs may bestretched with the stretchers and techniques described as follows:

1. Diagonal Intermeshing Stretcher

The diagonal intermeshing stretcher consists of a pair of left hand andright hand helical gear-like elements on parallel shafts. The shafts aredisposed between two machine side plates, the lower shaft being locatedin fixed bearings and the upper shaft being located in bearings invertically slidable members. The slidable members are adjustable in thevertical direction by wedge shaped elements operable by adjustingscrews. Screwing the wedges out or in will move the vertically slidablemember respectively down or up to further engage or disengage thegear-like teeth of the upper intermeshing roll with the lowerintermeshing roll. Micrometers mounted to the side frames are operableto indicate the depth of engagement of the teeth of the intermeshingroll.

Air cylinders are employed to hold the slidable members in their lowerengaged position firmly against the adjusting wedges to oppose theupward force exerted by the material being stretched. These cylindersmay also be retracted to disengage the upper and lower intermeshingrolls from each other for purposes of threading material through theintermeshing equipment or in conjunction with a safety circuit whichwould open all the machine nip points when activated.

A drive means is typically utilized to drive the stationery intermeshingroll. If the upper intermeshing roll is to be disengageable for purposesof machine threading or safety, it is preferable to use an antibacklashgearing arrangement between the upper and lower intermeshing rolls toassure that upon reengagement the teeth of one intermeshing roll alwaysfall between the teeth of the other intermeshing roll and potentiallydamaging physical contact between addenda of intermeshing teeth isavoided. If the intermeshing rolls are to remain in constant engagement,the upper intermeshing roll typically need not be driven. Drive may beaccomplished by the driven intermeshing roll through the material beingstretched.

The intermeshing rolls closely resemble fine pitch helical gears. In thepreferred embodiment, the rolls have 5.935″ diameter, 45° helix angle, a0.100″ normal pitch, 30 diametral pitch, 14½° pressure angle, and arebasically a long addendum topped gear. This produces a narrow, deeptooth profile which allows up to about 0.090″ of intermeshing engagementand about 0.005″ clearance on the sides of the tooth for materialthickness. The teeth are not designed to transmit rotational torque anddo not contact metal-to-metal in normal intermeshing stretchingoperation.

2. Cross Direction Intermeshing Stretcher

The CD intermeshing stretching equipment is identical to the diagonalintermeshing stretcher with differences in the design of theintermeshing rolls and other minor areas noted below. Since the CDintermeshing elements are capable of large engagement depths, it isimportant that the equipment incorporate a means of causing the shaftsof the two intermeshing rolls to remain parallel when the top shaft israising or lowering. This is necessary to assure that the teeth of oneintermeshing roll always fall between the teeth of the otherintermeshing roll and potentially damaging physical contact betweenintermeshing teeth is avoided. This parallel motion is assured by a rackand gear arrangement wherein a stationary gear rack is attached to eachside frame in juxtaposition to the vertically slidable members. A shafttraverses the side frames and operates in a bearing in each of thevertically slidable members. A gear resides on each end of this shaftand operates in engagement with the racks to produce the desiredparallel motion.

The drive for the CD intermeshing stretcher must operate both upper andlower intermeshing rolls except in the case of intermeshing stretchingof materials with a relatively high coefficient of friction. The driveneed not be antibacklash, however, because a small amount of machinedirection misalignment or drive slippage will cause no problem. Thereason for this will become evident with a description of the CDintermeshing elements.

The CD intermeshing elements are machined from solid material but canbest be described as an alternating stack of two different diameterdisks. In the preferred embodiment, the intermeshing disks would be 6″in diameter, 0.031″ thick, and have a full radius on their edge. Thespacer disks separating the intermeshing disks would be 5½″ in diameterand 0.069″ in thickness. Two rolls of this configuration would be ableto be intermeshed up to 0.231″ leaving 0.019″ clearance for material onall sides. As with the diagonal intermeshing stretcher, this CDintermeshing element configuration would have a 0.100″ pitch.

3. Machine Direction Intermeshing Stretcher

The MD intermeshing stretching equipment is identical to the diagonalintermeshing stretch except for the design of the intermeshing rolls.The MD intermeshing rolls closely resemble fine pitch spur gears. In thepreferred embodiment, the rolls have a 5.933″ diameter, 0.100″ pitch, 30diametral pitch, 14½° pressure angle, and are basically a long addendum,topped gear. A second pass was taken on these rolls with the gear hoboffset 0.010″ to provide a narrowed tooth with more clearance. Withabout 0.090″ of engagement, this configuration will have about 0.010″clearance on the sides for material thickness.

4. Incremental Stretching Technique

The above described diagonal, CD or MD intermeshing stretchers may beemployed to produce the incrementally stretched film or laminate ofnonwoven fibrous web and microporous-formable film to form themicroporous film products of this invention. For example, the stretchingoperation may be employed on an extrusion laminate of a nonwoven fibrousweb of staple fibers or spun-bonded filaments and microporous-formablethermoplastic film. The laminate of nonwoven fibrous web andmicroporous-formable film is incrementally stretched using, forinstance, the CD and/or MD intermeshing stretcher with one pass throughthe stretcher with a depth of roller engagement at about 0.060 inch to0.120 inch at speeds from about 550 fpm to 1200 fpm or faster. Theresults of such incremental or intermesh stretching produces laminatesthat have excellent breathability and liquid-barrier properties, yetprovide superior bond strengths and soft cloth-like textures.

The following examples illustrate the method of making microporous filmand laminates of this invention. In light of these examples and thisfurther detailed description, it is apparent to a person of ordinaryskill in the art that variations thereof may be made without departingfrom the scope of this invention.

EXAMPLE I

A microporous formable formulation containing 55% calcium carbonate, 36%homopolymer polypropylene (PP), 5% low density polyethylene (LDPE), 3%titanium dioxide master batch and 1% of combined processing master batchand antioxidant additives typically used in the film processing isextruded by a conventional single screw extruder 21 and a slot die 22 toform a web 26 which is about 450-500° F. The web 26 is leading into apressure nip formed by a rubber roller 25 and a metal roller 24. Therubber roll is partially immersed in a water tank (not shown), so itssurface is cooled by the water where the water is typically controlledat a temperature between 60-140° F. The web 26, after passing the firstpressure nip formed by rubber roller 25 and metal roller 24, isimmediately stretched in the machine direction between the firstpressure nip and the second pressure nip formed by the rubber roller 28and metal roller 27. The stretching ratio (R) is defined by the speed(V₁) of the first metal roller 24 and the speed (V₂) of the second metalroller 27. The stretch or roll gap (x) is defined by the narrowestdistance between the circumferences of first metal chill roller 24 andsecond metal roller 27 which can be directly measured by any feelergauge easily available. Table I demonstrates the experimental conditionsand the results for Examples I-O to I-D. I-O is a comparative examplewhere the speed of the first roller 24 and second roller 27 is the same(90 fpm) to make an unstretched film which is not microporous. Incontrast, Example I-A to I-D are stretched. The surface topology ofunstretched (Example I-O) and stretched (Examples I-A to I-D) are shownin FIGS. 4-8, identified as 1-O to 1-D.

Table 1 also demonstrates the benefits of this invention where the filmis extruded without draw resonance at an original gauge of 85 g/m² andis stretched to 50 g/m² (comparing Examples I-O and I-D). The speed atthe first roller (V₁) is 90 fpm and is 180 at the second roller (V₂) fora stretch ratio of 2:1. More broadly, this invention can achieve thinfilms which are up to 4 or more times thinner than the original extrudedfilm without draw resonance.

TABLE 1 First Metal Roller Second Metal Roller Roll Gap Stretch RatioFilm MVTR Temp Speed Temp Speed (x) (R) Thickness E-96E Mocon Example (°F.) (fpm) (° F.) (fpm) (inch) (v_(2/)v₁) (g/m²) g/m²/day g/m²/day I-0 9090 70 90 0.05   1:1 85 0 0 I-A 90 90 70 135 0.05  1.5:1 75 2794 4395 I-B136 90 70 158 0.05 1.75:1 58 2840 4305 I-C 139 90 70 167 0.05 1.85:1 522848 4765 I-D 139 90 70 180 0.05  2.0:1 50 2908 5237 Note 1: ASTM E-96Eis measured at 100° F. and 90% RH Note 2: Mocon MVTR is measured at37.8° C.

EXAMPLES II

A microporous formable formulation containing 52.8% calcium carbonate,38.8% LLDPE, 3% LDPE, 4.4% TiO₂ and 1% of processing aid master batchand antioxidant additives typically used in the film extrusion isextruded in a very similar manner as described in Examples I-O to I-D.The experimental conditions and the results are shown in the Table 2.The surface topologies of these examples are shown in FIGS. 9-11identified as II-A, II-B and II-C.

Table 3 shows the mechanical properties of Example II-B as MDO stretchedfollowed by incremental stretching in the MD and CD with the apparatusof FIG. 1 using intermeshing rollers 40, 41 and 42, 43 for stretching inthe CD and MD, respectively. The intermeshing of stretching rollers areat ambient temperature and the engagement of the rollers on CD and MDare 0.050″ and 0.040″, respectively.

TABLE 2 First Metal Roller Second Metal Roller Roll Gap Stretch RatioFilm MVTR Temp Speed Temp Speed (x) (R) Thickness E-96E Mocon Example (°F.) (fpm) (° F.) (fpm) (inch) (v_(2/)v₁) (g/m²) g/m²/day g/m²/day II-A125 100 70 200 0.050 2:1 54 4373 14,246 II-B 140 100 70 400 0.05 4:1 334540 15,219 II-C 140 100 70 400 1 4:1 35 4578 18,403

TABLE 3 Basis Tensile Strength Elongation at Break Weight MVTR (g/cm)(%) Example (g/m²) E-96E MD CD MD CD II-B as is 32.9 4373 2038 144 51489 II-B with 30.6 4259 1922 140 50 421 intermeshing

EXAMPLES III

Table 4 illustrates Examples I-D and III-A and III-B where the MDOstretched film of I-D was subsequently incrementally stretched in the CDand MD under conditions as shown in the Table 4 for the CD and MDstretchers described above.

Table 5 demonstrates the mechanical properties of the Table 4 products.The MDO stretching and interdigitating stretching produced microporousfilm with adjusted and balanced properties such as micropore pore sizes,MVTR, MD and CD modulus and MD and CD tensile strength, as shown byTable 5. The MDO microporous films were continuously CD interdigitated,and CD interdigitated and MD interdigitated, respectively. The surfacetopologies of these examples are shown in FIGS. 12-13, identified asIII-A and III-B.

TABLE 4 First Metal Roller Second Metal Roller Roll Gap Stretch RatioIntermeshing Depth MVTR Film Temp Speed Temp Speed (x) (R) CD MD E-96EWeight Example (° F.) (fpm) (° F.) (fpm) (inch) (v_(2/)v₁) (inch) (inch)g/m²/day (gsm) I-D 139 90 70 180 0.05 2:1 0 0 2908 50 III-A 139 90 70180 0.05 2:1 0.05 0 3362 45 III-B 139 90 70 180 0.05 2:1 0.05 0.04 383737

TABLE 5 Tensile Strength Elongation at Tensile @2% (g/cm) Break (%)Elongation (g/cm) Example MD CD MD CD MD CD I-D 1418 464 253 413 365 177III-A 1027 593 296 322 282 47 III-B 1106 421 218 294 279 63

EXAMPLE IV

A microporous formable formulation having an ABA coextruded structurewas extruded into a film with the MDO process where (A) contained 55.6%CaCO₃, 36.90% homopolymer polypropylene, 5.4% LDPE, 2% TiO₂ and 0.1% ofantioxidant additive and (B) contained 52.8% CaCO₃, 39.2% LDPE, 3.5%LDPE, 4.4% TiO₂ and 0.1% antioxidant additive. The roll gap was at 0.01inch and the stretching ratio was 1.0. The MDO process was followed withCD intermeshing at 0.055 inch and further followed with MD intermeshingat 0.045 inch using the CD/MD stretcher described above (see ExampleIV-A of Table 6). A microporous film was produced with a MVTR of 1300(ASTM E96E) grams/m²/day and a film weight of 21 grams per square meter(gsm) with a MD tensile at 25% of 218 g/cm. Then, the same ABAcoextrusion was run, but the stretching ratio was increased from 1.0 to1.25, 1.5, 1.75 and 2.0 while all other process conditions were kept thesame (see Examples IV-B, C, D and E of Table 6). The film gauge wasreduced from 21 gsm to 19.3, 16.5, 15 and 13.95, respectively. While thefilm gauge was reduced, the 25% MD tensile strength increased from 218gsm to 240, 265, 311 and 327 gsm, respectively. This was very beneficialto web handling of the thin gauged film. While 25% MD tensile wasincreased, the 25% CD tensile decreased from 142 gsm to 118, 82, 58 and54 gsm. This provided soft hand feel film, as well. These types ofmicroporous films are suitable for making diaper and feminine carenapkin outcover while providing high strength on MD direction forproduct high speed conversion, yet providing soft product for end use.Table 7 is provided to show the mechanical properties normalized to 21gsm for same gauge comparison. Accordingly, microporous film productsproduced by the MDO process of this invention can be followed with CDand MD intermeshing to provide a film with its tensile strength at 25%elongation of MD to CD ratio of greater than 2.

Table 6 examples also demonstrate original gauge reduction of theextruded film without draw resonance at V₁ from 21 gsm (1V-A) to 13.9gsm at V₂ (IV-E), nearly two times thinner. The speed at V₁ was 325 fpmand 650 at V₂ for a stretch ratio of 2.

TABLE 6 First Tensile Metal Roller Second Metal Roller Roll Gap StretchRatio Incremental Stretching Tensile MD CD Temp Speed Temp Speed (x) (R)CD MD Film Wt. MVTR at 25% at 25% Example (° F.) (fpm) (° F.) (fpm)(inch) (v_(2/)v₁) (inch) (inch) (gsm) E-96E (g/cm) (g/cm) IV-A 148 32570 325 0.01 1 0.055 0.045 21 1300 218 142 IV-B 148 325 70 406 0.01 1.250.055 0.045 19.3 1400 240 118 IV-C 148 325 70 488 0.01 1.5 0.055 0.04516.5 1400 265 82 IV-D 148 325 70 569 0.01 1.75 0.055 0.045 15 1400 31158 IV-E 148 325 70 650 0.01 2 0.055 0.045 13.9 1450 327 54 Note 1: Filmweight gsm is gram per square meter Note 2: MVTR is ASTM E96E ingrams/m²/day Note 3: Tensile MD at 25% is the MD tensile strength at 25%elongation Note 4: Tensile CD at 25% is the CD tensile strength at 25%elongation

TABLE 7 Tensile Strength at MD direction Tensile Strength at CDdirection measured at different elongation measured at differentelongation Ultimate Ultimate Film Wt. 10% (g/cm) 25% (g/cm) (g/cm)/break(%) 10% (g/cm) 25% (g/cm) (g/cm)/break (%) Sample (gsm) As is At 21 gsmAs is At 21 gsm As is As is At 21 gsm As is At 21 gsm As is IV-A 21 172172 218 218 407/207 106 106 142 142 196/340 IV-B 19.3 171 186 240 261457/339 79 86 118 128 166/313 IV-C 16.5 179 228 265 337 422/210 55 70 82104 130/326 IV-D 15 187 262 311 435 452/118 39 55 58 81  87/316 IV-E13.9 185 280 327 494 516/142 36 54 54 82  96/345 Note 1: “As is” is themechanical properties of the film weight as it is Note 2: “At 21 gsm” isthe mechanical properties normalized to 21 gsm for same gauge comparisonNote 3: “Ultimate (g/cm)/break (%)” are the Ultimate tensile strength(in grams/cm) at the elongation at break (in %)

EXAMPLE V

A microporous formable formulation containing 52.8% CaCO₃, 39.2% LLDPE,3.5% LDPE, 4.4% TiO₂ and 0.1% antioxidant additives was extruded into afilm and stretched at a gap greater than 1. At any stretching ratio, auniformly stretched microporous film cannot be attained. Big areas ofunstretched zones cannot be eliminated (see FIG. 14 photos for ExamplesV-A and V-B). However, at a roll gap at 1″, microporous films withuniformly stretched appearance are achieved at a stretch ratio of 4:1(see FIG. 14 photos for Examples V-C and V-D). With the roll gap reducedto 0.05 inch, uniform microporous films can be achieved as low as1.25-1.50 as shown in FIG. 14 photos V-C and V-D. According to thisExample V, a short stretching or roll gap at 1″ or below is critical fora successful practice of this invention.

FIG. 14 illustrates the actual products of Example V which were producedto show the criticality of the short gap method and apparatus of thisinvention in order to provide acceptable microporous film products ofsubstantially uniform gauge. Accordingly, “substantially uniform gauge”is meant to define those microporous film products where the degree ofuniformity is such that no “tiger striping” is visually apparent to thenaked eye and essentially complete uniform stretching of the product hasbeen achieved with reference to sketches C and D of Example V in FIG.14. Sketches A and B of actual products of Example V demonstrateunacceptable products where the tiger stripes are very visible to thenaked eye which makes the product unsightly or ugly. In the case ofproduct sketches A and B, the unstretched widths of tiger stripes aregreater than ⅛ inch and are very visible to the naked eye, predominatelyover a significant portion of the product which makes the productunsightly or ugly. However, where the tiger stripes have been reduced orcompletely eliminated, as is the case in sketches C and D, acceptableproduct has been produced by uniform stretching to provide substantiallyuniform gauge. The fine lines as represented by sketch C are so fine, onthe order of less than 1/32 inch that they do not detract from the eyeappeal of the product or such lines do not exist at all as in the caseof sketch D. Accordingly, one of ordinary skill in the art willunderstand that the objectives of this invention are accomplished byemployment of the short gap to produce substantially uniform gauge asrepresentative of the elimination of tiger stripes which render theproduct unacceptable, as represented by the comparative examples ofExample V.

EXAMPLE VI

A microporous formable formulation having an ABA coextruded structurewas extruded into a film with the MDO process where (A) contained 52.8%CaCO₃, 39.2% LLDPE, 3.5% LDPE, 4.4% TiO₂ and 0.1% antioxidant, and (B)contained 90% polypropylene polymer and 10% polyethylene polymer. Theroll gaps were at 0.02 inch and 0.035 inch with stretching ratios at3.0, 4.0 and 5.0. Films were produced with microporous layers on bothsides and non-microporous layer in the middle. The resulted films havevery low moisture vapor transmission rate equivalent to typical thinfilms of polyolefins, but provide microporous film surface for printing,nice hand feel, etc., which are suitable for many packagingapplications. The following Table 8 shows the results.

TABLE 8 First Metal Roller Second Metal Roller Roll Gap Stretch RatioTemp Speed Temp Speed (x) (R) Film Wt. MVTR Example (° F.) (fpm) (° F.)(fpm) (inch) (v₂/v₁) (gsm) E-96E VI-A 120 120 70 360 0.02 3.0 32.2  0-45VI-B 140 120 70 480 0.02 4.0 34.5 30-90 VI-C 110 120 70 480 0.035 5.033.5 15-75 Note 1: Film weight gsm is gram per square meter Note 2: MVTRis ASTM E96E in grams/m²/day

EXAMPLE VII

A 100% polyethylene resin is extruded into a single layer film by usingthe MDO process of this invention at a roll gap of 0.045 inch. A filmhas been produced using this technique at more than 1000 fpm. Theproduct of this example has a substantially uniform gauge of 27 gsm andis nonmicroporous. Accordingly, the terms “substantially uniform gauge”are also meant to define those nonmicroporous films of high quality inuniform gauge or thickness. Table 9 describes the results.

TABLE 9 First Metal Roller Second Metal Roller Roll Gap Stretch RatioTemp Speed Temp Speed (x) (R) Film Wt. MVTR Example (° F.) (fpm) (° F.)(fpm) (inch) (v₂/v₁) (gsm) E-96E VII 120 499 70 1025 0.045 2.05 27 0-30Note 1: Film weight gsm is gram per square meter Note 2: MVTR is ASTME96E in grams/m²/day

Accordingly, the above Examples I-VII demonstrate that the method ofthis invention produces thermoplastic films at high speeds with a shortroll gap where films are of substantially uniform gauge and are eithernon-microporous, breathable microporous, or non-breathable microporous.

The above data and experimental results demonstrate the uniquethermoplastic films and method and apparatus of this invention forstretching said thermoplastic films. Other variations will be apparentto a person of skill in the art without departing from this invention.

1. An apparatus for making a stretched thermoplastic film comprising anextruder for extruding a thermoplastic molten extrudate in the form of aweb, a chill roller for operating at a peripheral velocity (V1) and at atemperature (T1) to receive and cool said web thereby forming a film, asecond roller for operating at a peripheral velocity (V2) greater thansaid V1 to receive said film at a temperature (T2), said second rollerspaced from said chill roller to provide a roll gap of no more than oneinch between said chill roller and said second roller for stretchingsaid film to provide a film of uniform gauge.
 2. The apparatus of claim1 wherein said chill roller is spaced from said second roller to providea roll gap of about 0.005 to about 0.05 inch.
 3. The apparatus of claim1 wherein said rollers are designed for operating at a ratio ofvelocities of V2 to V1 between about 1.25:1 and about 4:1.
 4. Theapparatus of claim 1 wherein said chill roller is a metal roll whichcooperates with a backing roller to form a first nip for receiving saidweb.
 5. The apparatus of claim 4 wherein said chill roller is selectedfrom the group of an embossing metal roll and a flat chrome roll.
 6. Theapparatus of claim 4 wherein the backing roller is selected from thegroup of a rubber roll a metal roll.
 7. The apparatus of claim 1 whereinsaid second roller cooperates with another backing roller to form asecond nip for receiving said film from said first nip.
 8. The apparatusof claim 7 wherein said second roller is selected from the group of ametal roll a rubber roll.
 9. The apparatus of claim 1 wherein a vacuumbox is located adjacent to said chill roller.
 10. The apparatus of claim9 wherein said chill roller is selected from the group of an embossingmetal roll and a flat chrome roll.
 11. The apparatus of claim 9 whereinsaid second roller cooperates with a backing roller to form a first nipreceiving film from said chill roller.
 12. The apparatus of claim 1further comprising incremental stretching rollers for incrementallystretching said film.
 13. The apparatus of claim 12 wherein saidincremental stretching rollers are comprised of a first section and asecond section for incrementally stretching the film in a firstdirection followed by incremental stretching in a second direction. 14.The apparatus of claim 13 wherein said first and second stretchingdirections are substantially perpendicular to one another.
 15. Theapparatus of claim 1 wherein said extruder operates up to a maximumextrusion rate and said chill and second rollers operate at V1 and V2,respectively, up to about 4 or more times said extrusion rate.
 16. Theapparatus of claim 15 wherein said rollers operate at speeds up to about4000 or more linear feet per minute.